Sulfur oxide (SO.sub.x), which is generated from the combustion of more than 200 kinds of organic sulfur compounds contained in petroleum, causes not only air pollution but also acid rain and exerts a bad influence on the ecosystem. In this connection, the desulfurization is essential in order to reduce the sulfur contents in fossil fuel. For the desulfurization of petroleum products, the hydrodesulfurization has been conventionally carried out by adding hydrogen at the high temperature of 300-360.degree. C. and high pressure of 35-170 atm.
However, in case of the more complex compounds than dibenzothiophene ("DBT"), which constitutes more than 30% of whole sulfur-containing organic materials in fossil fuel, the hydrodesulfurization is no more effective, and deep desulfurization has been regarded as an alternative way of removing sulfur from these materials, though it depends on the nature of crude oil and its composition.
Under the circumstances, there are strong reasons for exploring and developing a novel desulfurization method which consumes little energy, since the chemical methods such as the hydrodesulfurization and deep desulfurization have been proven less satisfactory in terms of the economy of maintenance, exhaustion of low sulfur-containing crude oil and environmental destruction.
Unlike the prior art chemical methods, biological desulfurization can be carried out at the room temperature and atmospheric pressure. In this regard, desulfurizing bacteria has been utilized for saving the cost for the installation and operation, when compared with previous chemical desulfurization using hydrogen. Especially, the biological method has a merit in a sense that the organic sulfur compounds which are more complex than DBT can be readily desulfurized.
In general, biological desulfurization of fossil fuel using microorganisms may be carried out under the aerobic or anaerobic condition. Accordingly, the desulfurization using aerobic bacteria has advantages as followings: It can be operated in an efficient and speedy manner; it does not consume expensive hydrogen; and, it does not require additives or equipments to maintain the electrical reducing potential and the anaerobic state.